Well of course, you would change a file's atime (to "now") by opening it for read access and reading some data from it; to change the mtime (to "now") you'd have to open for write access and then actually modify its contents somehow (i.e. actually write something to it). The only way I can imagine to set ctime (to "now") is to delete the file and then (re)create it.

There's a unix shell command called "touch" that allows a bit more flexibility: it can be used to set atime and/or mtime -- either to "now" or to a specified date/time (but setting things to a future date might be a bad idea). The man page for "touch" doesn't say anything about altering the ctime of a file.